Allow user to indicate pasted content source

[Dumluregn]

Type of report

Feature request

Provide description of the new feature

During development of PfW and its peers we encounter a number of issues caused by our inability to easily detect the source (and therefore sometimes the format) of pasted data. That makes us use more or less ugly hacks to ensure the outcome will look as expected by user.

Instead we could give a minimal dose of responsibility to the user and give him the possibility to indicate the source of content so that we could handle it properly. That could save us time, lines of code, unexpected bugs and regressions (whoa, seems like a lot of profits). For now implementation is an open topic - we just know that it should be as user-friendly as possible.

Current ideas are as follows:

1. Balloon toolbar displaying after pasting content allowing user to change source and compare the outcomes, similar to Word solution:

CC @Comandeer as you have already created something like this during one of company hackathons - please show us what you've got :)

2. Splitbutton (Unify pasting UI #3276):
   
   There was also a broader discussion about Splitbutton plugin itself in Split button UI component #1679.

3. Dialog displaying after detecting ambiguous content with options to choose from.

Sources for user to choose from may be:

• MS Office (should it be divided into Word, Excel, PowerPoint?)
• GDocs
• LibreOffice (Introduce paste from libre office plugin #3624)
• Paste as simple text
• Other (e.g. content pasted from web browser)

Some important insights about handling pasting from different sources are also in #3258.

[Comandeer]

CC @Comandeer as you have already created something like this during one of company hackathons - please show us what you've got

The solution is available on https://github.com/ckeditor/ckeditor4/tree/hackathon-pastebutton

Personally I'm for joining 1 & 2. However I'm more for one paste button with all options, not two.

[f1ames]

This will also allow use enabling PfLO in Safari (see #3809 (comment)).